IRAN has vowed to resist Western pressure to make what it regards as excessive concessions over its nuclear programme, highlighting obstacles that could prevent an historic deal being reached by a November 24 deadline.

US Secretary of State John Kerry declined to make any predictions for what he called a "critical week", during which negotiators from Iran and six world powers will push to end a 12-year dispute over Iran's nuclear activities and dispel fears of a new Middle East war.

After nearly a year of diplomacy, they aim to reach a settlement at the talks in Vienna that would curb Iran's atomic activities in return for phasing out sanctions that have severely hurt its economy.

However, Iranian and Western officials have said next Monday's self-imposed deadline is unlikely to be met, and an extension is the most likely outcome. They say it is possible to agree the outline of a future accord, but it would take months to work out the details.

Iranian Foreign Minister Mohammad Javad Zarif met former European Union foreign policy chief Catherine Ashton, who is coordinating the negotiations, in the Austrian capital yesterday. It was unclear when the seven states involved would formally start the talks.

Mr Kerry said: "This is a very critical week. It's imperative that Iran works with us in all possible efforts to prove to the world the (nuclear) programme is peaceful."

Foreign Secretary Philip Hammond said: "I believe a deal can be done but Iran needs to show more flexibility if we are to succeed."

Six states - France, China, Russia, Germany, the US and UK - want Iran to scale back its capacity to refine uranium.